1. Field of the Invention
This invention relates generally to automated pressure control and, more specifically, to an Improved Pressure Controller and Method.
2. Description of Related Art
The term, xe2x80x9csemiconductor processing equipment,xe2x80x9d refers to a seemingly infinite variety of large, highly expensive pieces of machinery that are used to conduct a variety of different processes that ultimately result in a completed semiconductor device. What is a common design aspect for many pieces of semiconductor processing equipment is the need for accurate, fast and reliable pressure control of the vacuum within the chamber where the process is taking place. If we look at FIG. 1, we can review how a conventional semiconductor processing tool system 10 is arranged today.
FIG. 1 is a depiction of a conventional semiconductor processing tool system 10. As shown in FIG. 1, the processing tool 12 is typically supplied by gas that is transmitted from a gas supply 14 (such as the bottle shown) through a gas supply line 16 until it gets to the vicinity of (or inside of) the semiconductor processing tool 12, where the actual flow of the gas to the chamber is controlled by a mass flow controller 18. In this way, the tool 12 can regulate when and how much gas to inject into the processing chamber 20.
There is generally a chamber pressure sensor 22 that provides an external signal via the pressure signal conduit 34. This external pressure signal typically can be either analog or digital in form, and represents the pressure conditions within the chamber 20. The signals are carried by a pressure signal conduit 34 to a conventional pressure control means 30. Within the pressure control means 30, the pressure signal is generally summed with a host tool logic signal later referred to as host tool pressure setpoint. The host tool pressure setpoint is generally generated by the tool logic controller 32, with its content being an analog or digital pressure setpoint value. These tool logic signals are transmitted to the pressure control means 30 by a tool logic signal conduit 36.
If we refer back to the tool 12, we can also see that another important feature that is many times found within the tool 12 is a plasma generator unit 23. This feature is important since plasma generators create sudden and sometimes large pressure deviations. Plasma generators essentially energize the gas molecules which splits them into ionized atoms and species. These ionized species are much more reactive than their molecular xe2x80x9cparentsxe2x80x9d thus greatly speeding up and increasing the selectivity of processes such as etch and deposition. The instant the plasma is turned on, a fraction of the gas molecules split in to pieces thereby producing instant undesirable increases in chamber pressure. Similarly, the supply lines 16 (and the gas they transmit) also have an effect on the pressure within the chamber 20. The chamber 20 is generally kept in a vacuum state in order to prevent impurities from contaminating the semiconductor process. The conventional arrangement for maintaining the vacuum condition in the chamber 20 is via a vacuum source 24, such as the vacuum pump 24 shown. The vacuum pump 24 simply pumps to an exhaust 25 while drawing a vacuum on a vacuum transmission line 26. Between the vacuum source 24 and the vacuum transmission line 26 is found a valve 28. It is by actuation of this valve 28 that the pressure can be raised and lowered (usually in the sub-atmospheric range) within the chamber 20.
Once the pressure signal and tool logic signal are summed in the pressure control means 30, the resulting signal is sent to a motor driver circuit 42 via an external valve command conduit 38. This conduit 38 is either hard wired via conventional cable, printed circuit board trace, or wire, however, it could also be wireless. The motor driver circuit 42 is actually a sub-component of a valve control assembly 40. The other components of the valve control assembly 40 are an internal valve command conduit 44 and a motor/valve drive assembly means 46 for actuating the valve 28. As should be appreciated, the signals generated by the pressure control means 30 are acted upon by the valve control assembly 40 to open and close the valve 28 such that the pressure in the chamber 20 is regulated. As described above, the pressure control system is influenced by external factors called states of the process, in particular, the turning on and off of gas inputs to the chamber and the initiation of RF events to create plasma are primary contributing factors. The pressure control algorithm (executed by the Pressure Control Means 30) constantly works at maintaining the pressure regulated at the required value by actuating the valve in order to compensate and balance the pressure responsive to the changing states of the process. It is clear that the pressure regulation task can be performed only as well as the individual elements comprising the closed loop system permit. As such the valve control assembly (40) is an essential component in terms of its accuracy and speed of response to maintain quality and/or stability of the control system. If we now turn to FIG. 2, we can look more closely at the valve control assembly 40 of the conventional system.
FIG. 2 depicts a conventional valve control assembly 40. As can be seen, the resultant signal of the summed commands from the pressure control means 30 in FIG. 1 arrive at the motor driver circuit 42 via an external valve command conduit 38. As discussed above, this is typically a cable that is run for whatever length necessary to extend between the pressure control means 30 and the motor driver circuit 42. Between the motor driver circuit 42 and the motor/valve drive assembly means 46 is an internal valve command conduit 44. In the conventional system, this conduit, too, is an external cable running between the motor driver circuit 42 and the motor/valve drive assembly means 46. The motor/valve drive assembly means 46 conventionally comprises a motor drive 48 such as a conventional stepper motor, which in turn drives a required reduction gear, or other means of mechanical advantage 52 via a motor shaft 50. In other forms, the motor drive 48 is connected to a valve stem 54 via belts and pulleys. In any case, it is conventional in the art that there not be a direct connection or coupling between the motor drive 48 and the valve means 28 without some method of mechanical advantage or reduction gearing having the effect of increasing the number of revolutions of the motor drive 48 needed to create a full open to close cycle of the valve means 28. This mechanical advantage typically also has the beneficial effect of increasing the step resolution as many folds as the reduction factor of the mechanical reducer means. However, it also represents an actuation speed penalty of the same magnitude, as the motor has to travel farther for the same valve displacement. Additionally, the increased resolution is partially absorbed and degraded by the inherent nonlinearity (backlash) introduced by the mechanical reducer means. That actuation speed handicap has proved to be more detrimental to the quality of the pressure control dynamic characteristics and transient response performance than initially expected. A further note is that within the conventional internal valve command conduit 44, there is typically one single unidirectional path that extends from the motor driver circuit 42 to the motor drive 48 with the exception of two limit switches that are normally used within the motor valve drive assembly to reference the open and closed valve positions. These switches return a binary logic signal that cannot resolve position continuously across the stroke of the valve but only at two discrete locationsxe2x80x94in order to distinguish these limit-switch-generated signals from signals to be discussed later on in connection with FIG. 4, we shall refer to these signals as xe2x80x9cstroke reference feedback signals.xe2x80x9d We will refer to this path as the command leg 56. The command leg 56, again, is unidirectional (excluding the stroke reference feedback signals), and only extends from the motor driver circuit 42 to the motor drive 48, and not vice versa. If we now turn to FIG. 3, we can examine how the conventional chamber pressure control process 300 operates.
We will start with the host tool pressure setpoint signal 302 arriving at the pressure control means 30. The pressure control means further comprises summing junction means 31 for the pressure sensor signal 314 to be compared with the host tool pressure setpoint signal 302 and generate a pressure error signal 304. That error signal is operated on by a pressure control algorithm 303 to produce a pressure control signal 306 that represents the desired change in valve position intended to correct said pressure error. If the system incorporates a conventional step motor drive, the pressure control signal 306 is transmitted from pressure control means 30 to the motor driver circuit 42 where it is converted to a position control signal 310. This signal 310 is then transmitted to the motor drive assembly means 46. Valve motion 312 is generated by actuating the valve stem 54. The valve stem 54 accordingly opens or closes the valve means 28 which, in turn, reduces or increases the conductance of the vacuum transmission line 26. This will respectively result in an increase or decrease in pressure within the processing chamber 20xe2x80x94a quantity that is continuously monitored by the pressure sensor 22. The monitored pressure is used to generate a pressure sensor signal 314 which is fed back to and again compared with the host tool pressure setpoint 302 by the summing junction 31. This above defined closed loop will herein be referred to as the pressure control loop. In practice the implementation of the pressure control loop is executed with electronics incorporating both discrete and continuous signals and is repeated in an iterative fashion.
As can be seen here, the vacuum transmission line 26, the processing chamber 20 and the chamber pressure signal 316 are all depicted in dashed lines; this is to highlight the fact that the position of the valve is not the only condition to affect the chamber pressure. Because of numerous external factors such as the turning on and off of gas inputs to the chamber and the initiation of RF events, the stability of the process is often challenged or disturbed. The efficiency with which these disturbances can be handled or rejected is substantially dependent on the accuracy with which the valve drive means can be rapidly and efficiently operated. In that context the remaining portion of this application will be devoted to illustrating the advantage of a system that provides nested closed-loop position control of the motor drive assembly means 46 by the motor drive circuit means 42. This is implemented specifically to minimize the chamber pressure sensitivity to process variations and better exploit the pressure feedback information thus enhancing the pressure control performance.
In light of the aforementioned issues and fundamental shortcomings associated with the prior systems and methods, it is an object that the present invention provide a method that allows for greater quality and accuracy of control resulting in both faster times to setpoint and better steady state pressure stability. The preferred invention will rely on an enhanced valve control scheme that integrates a valve position servo control system nested within the conventional pressure control loop. In other words, it is a further object that the pressure control function be accomplished by generating a pressure control signal in terms of valve position. That control signal would in turn be transformed into an actual valve position by a valve/motor drive feedback system. In contrast with prior art systems that make use of open loop motor control, closed loop motor control brings an overwhelming advantage to the pressure control function. One further object is to utilize the higher-resolution addressability of motion that allows for a conventional motor to be directly linked to the valve stem without a geared reducer thus enabling the valve to operate at a faster speed, and to further provide the improved positional precision that is achievable by closed loop operation. It is a still further object that the improved system relieve the pressure control function of the design constraints of low valve speed and limited accuracy of valve positioning.